Legacy data centers employ a hot aisle/cold aisle arrangement of the IT racks. The fronts of the racks face each other and draw cold air into the rack to cool rack mounted IT devices (i.e. servers, switches, etc.). Conversely, the rear sides of the rows of racks face one another, expelling the hot air into the hot aisle. The issue with hot aisle/cold aisle designs is that the air is free to move wherever it will.
Blog Approaches to Data Center Cold-Aisle Containment